Positive pressure respiratory therapies, such as continuous positive airway pressure (CPAP) therapy, and intermittent positive pressure ventilation therapy (IPPV), are commonly administered for the treatment of a wide range of respiratory conditions, including central and obstructive sleep apnea (OSA), chronic obstructive pulmonary disease (COPD), restrictive respiratory insufficiency and acute respiratory failure (ARF). Acute and chronic life support systems also make use of positive 15 pressure respiratory therapies using volume or pressure controlled cycles. Typically, positive pressure delivered via an externally fitted interface such as a removable, face mask, for example, would be used to provide intermittent positive pressure treatment of responsive conditions in a conscious user, that is, when the user is either awake or asleep, but otherwise fully arousable and able to sustain at least partial respiratory effort. Tracheostomy may be required for chronic life support involving, for example, total loss of respiratory muscle innervation. In temporarily anesthetised or otherwise unconscious users without sufficient self-supporting respiratory drive or those with unstable airways, ventilation would normally be accomplished with an invasive interface means, such as an endotracheal tube or pharyngeal mask, to ensure reliable connection of the pressure source and hence predictable ventilation. In general it is preferable to adopt the least invasive means to treat a specific condition in order to mitigate complications, complexity and cost of care. For this reason, the application of externally sealing face masks has become increasing popular in hospital settings where feasible. Examples include ventilation with full face or nasal masks for management of acute respiratory failure. Additional benefits of non-invasive positive pressure treatment methods include their improved applicability to use in a home setting where ease of use, comfort and efficacy are key factors in determining patient compliance with therapy. The ability to treat chronic disorders in the convenience of the home setting improves long-term health outcomes for users and relieves the burden on hospital resources.
Any gas pressure delivering means can be used as the source of positive pressure. In the prior art, a pressure source would typically comprise a gas flow generator and a gas flow circuit in the form of a flexible air delivery hose which connects the gas flow generator to a user interface. If a gas flow generator is made small enough it could be attached directly to, or integrated with, the user interface, and then it alone would comprise the positive pressure source. Where a primary gas flow delivery tube is used it may include a secondary tube or limb to recover or direct expired gas as may be used with anesthesia or ventilation. In the case of nasal CPAP therapy, a source of breathable gas maintained at a substantially constant treatment pressure above atmospheric pressure over a breathing cycle, is applied to a user's airway via a user interface, such as a nasal mask, mouth mask oronasal mask, nasal prongs or other externally fitted device. CPAP therapy is most commonly used to treat OSA in a home-care setting, although it is also applicable to a variety of other respiratory conditions. CPAP therapy, particularly for home use, may also include limited pressure release modes, whereby treatment pressure is reduced during exhalation and restored to its previous preexhalation level at the end of expiration. This approach offers less resistance to expiratory effort and is intended to either enhance breathing comfort in the case of sleep apnea, or, to assist or support a user's own respiratory effort in the case of respiratory insufficiency. The latter case is often termed bi-level treatment to reflect the fact that distinct pressures are applied to a user's airway during a single breathing cycle. Pressures may also be varied as a function of time and flow, in which cases there may be a range of pressure applied during a single breathing cycle. To facilitate venting of expired tidal volume from a user's airway when using an externally fitted user interface, it typically will have incorporated into its structure a vent or vents of fixed dimension which are open to atmosphere. In this way, a breathable gas is able to leak from the vents continuously under the effect of the positive pressure applied to the user interface from the pressure source. The magnitude of this vented flow of breathable gas to atmosphere will be a function of the pressure within the user interface and vent configuration. Typically, designers will attempt to find a suitable compromise between vent size and number, such that sufficient gas is vented in order to limit the amount of exhaled carbon dioxide (CO2) which is rebreathed by the user, whilst keeping the magnitude of vented gas low enough not to require a significant increase in pressure source capacity to compensate for this operational leak. For example, at a user interface pressure of 10 cm of water, the vent flow rate may be in the order of 20 liters/min whereas at 20 cm of water it may be in the order 60 liters/min. These figures assume that there is no additional unintended leak, as may be attributable to a poorly fitting user interface. An absence of means to physically prevent reverse flow of expired gas back towards the pressure source means that some fraction of a user's exhaled air may accumulate within the pressure source and thereby be re-inhaled during subsequent inhalation or lung reinflation. Additionally, some fraction of the air provided by the pressure source is directed straight to atmosphere via the vents without entering the user's airway since it must be applied to reducing the effective total dead space of the user including the interface and pressure source. It is apparent therefore that the lower the therapeutic pressure, the less effective is the venting of stale expired gas from a user's respiratory system. This last limitation in the current state of the an requires devices to specify a minimum operating pressure in order to provide a safe level of elimination of expired tidal volume and hence prevent rebreathing of CO2. Typically, this minimum pressure required from the pressure source is in the order of 4 cm of water. Prolonged use below this minimum pressure puts the user at risk of rebreathing a substantial proportion of their expired air and asphyxiation if both nose and mouth breathing routes are covered by a well-fitting oronasal mask, for example. It is evident that the prior an exhibits further inherent limitations depending on operating circumstances. At high rates of breathing cycles, deep breathing, or a combination of both over a prolonged period, there will be increased levels of rebreathing of expired air and CO2, thereby increasing the user's effective airway dead space, which includes the interface and pressure source. Furthermore, the capacity of the pressure source must be increased to compensate for continuously vented flow which necessarily increases with pressure as described. Additionally, when humidification is required by the user, further capability must be factored into the design of the pressure source because some varying fraction of the humidified air is vented to atmosphere through the vents without entering the user's airway, thereby requiring both heating and storage of an additional volume of water that is not used to humidify a user's airway. Similarly, if required, flow of instilled supplementary therapeutic gases, for example oxygen, or other therapeutic substances must also be correspondingly increased to compensate for loss due to continuous venting of the breathable gas.
When breathable gas exits the vents of the user interface, it typically creates noise which may irritate the user or their bed partner. The acoustic magnitude of this vent noise is proportional to the rate of vent flow. It can be further appreciated that exhaled gas combined with flow from the pressure source will exit the user interface with sufficient velocity and volume to increase the risk of spreading infectious particles if present in exhaled gas to the surrounding environment. This may pose a significant infection risk to health workers in a hospital setting and others in the vicinity. If the source of breathable gas fails to generate the prescribed minimum pressure, such as during a power failure, users fitted with a full face mask, such as one which covers both the nose and mouth, must also be fitted with an anti-asphyxia valve to ensure the user does not rebreathe a substantial part of their expired tidal volume which, in the absence of sufficient background pressure and corresponding flushing flow, will accumulate in the pressure source. A further application of the invention is hi-level therapy wherein, rather than administering a substantially constant positive pressure over a breathing cycle, pressure will be varied within a breathing cycle to assist natural breathing. As a general principle, pressure applied during lung filling or inspiration will be greater than that applied during lung emptying or expiration to facilitate gas movement into and out of a user's respirator system. Transition from a higher to lower pressure is most often triggered by machine sensing, of the user's breathing, or follows pre-set machine controlled breathing rates, pressures or volumes. Means of connecting a source of pressurised breathable gas to a user's facially accessible airway will involve a range of user interface devices similar to those described for CPAP therapy and corresponding methods of venting, expired air from a user's respiratory system, that is, through a series of small vents often positioned in the interface itself. Such arrangements will suffer similar limitations to those described previously. Additionally, it can be appreciated that CO2 rebreathing may produce a more detrimental impact in hi-level therapy users due to the fact that these individuals typically exhibit a greater degree of respiratory impairment by virtue of background hypoxia, hypercapnia, more rapid breathing and perhaps exaggerated tidal volume, particularly during acute exacerbations and their prodrome. Since the pressure applied during exhalation is lower than that applied during inhalation, the ability to clear residual expired air from the system may be further compromised. A clinical compromise involves finding a suitable range of pressures to facilitate user comfort, adequate ventilation and adequate flushing of exhaled gas accumulated within the pressure source to minimize negative impacts related to CO2 rebreathing. In yet a further application, positive pressure therapies, particularly those involving, ventilation, may also be administered via endotracheal tubes, laryngeal masks, tracheotomies or similar invasive means. In these circumstances, it is common to provide active venting wherein the venting, of exhaled tidal volume from a user's airway is provided through an arrangement of inspiratory and/or expiratory valves placed in the breathing circuit and often under automated synchronised control from the pressure source. The prior art describes many circuit arrangements depending on the clinical requirements, including open and fully closed recirculating systems. In its simplest for an active exhalation valve will be present to direct exhaled gas to atmosphere while allowing fresh breathable gas to be supplied to the user's airway on cycling to an upper pressure. Such control means add mechanical and electrical complexity and increased risk of asphyxiation by rebreathing in the event of failure of valve actuation systems and components.